<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    
</body>
<script>
    var a = 100;
    var b = 15;
    var c = "8";
    
    //如果加号两边都是数字，加号就是相加的意思；
    //如果加号两边有字符类型，加号就是拼接的作用；
    console.log(a + b);
    console.log(b + c);
    console.log(typeof(b + c));

    var x = 3;
    var y = "3";
    var z = 5;
    z += y;
    var a = y + z;
    document.write(x+y+'<br/>');   //33
    document.write(z+'<br/>');     //53
    document.write(a+'<br/>');     //353
    document.write(x+y+z+'<br/>'); //3353
</script>
</html>